The present invention is directed to a method of dewatering a subterranean space, especially an underground mine, which comprises collecting water artificially tapped from a water storage zone or collecting water of imbibition and transferring the water, through the use of a subterranean pumping station and at least one refill bore hole to a refill zone, said subterranean space being separated from said refill zone by an impermeable layer.
The most common way to dewater a mine is to collect the water tapped from the mine or the water of imbibition and to raise it by a pump to the surface. When the loss of the water balance in the layers tapped by mining has to be moderated, the water or part of the water raised from the mine is pumped back to the subterranean water storage zones, usually through injection wells.
The twofold technical task, namely to remove the water from the mine-field continuously and to return the stock of water abstracted from the subterranean water storage zone, is a well known prior art method which is complicated and expensive. Also, when reinjecting the water from the surface, the risk of contamination is enhanced.
It is an object of the present invention to eliminate the above insufficiencies by providing a method of removing the water from mine-fields and other subterranean spaces and returning it to the subterranean water-system at reduced cost and less risk of contamination than exists in the known methods.
The concept of the present invention is based on the perception that the water tapped from the subterranean water storage zone for protecting the subterranean space or the water of imbibition can be directed from the same subterranean space to a refill zone for maintaining the subterranean water balance if two conditions are met. The first condition is that there should be a zone of sufficiently high hydraulic resistance between the tapped subterranean space and the refilled zone of the subterranean water-system.
The second condition is that between the mine-fields and the refill zone of the subterranean water-system there should be an impermeable seam in which the water pressure causing the spontaneous hydraulic cracking exceeds the injection pressure in use.
If the first condition is not met by the refill zone, the water output of tapped water or the amount of water of imbibition is strongly increased.
If the second condition is not met, the water pressed back to the subterranean water-system will cause hydraulic cracking through the impermeable seam and thus the water of the subterranean water storage system will rush into the mine while it is being refilled.
According to the present inventive method of dewatering a subterranean space, especially a mine, the method comprises artificially tapping a water storage zone or collecting water of imbibition and transferring the water through a subterranean pumping station to a refill zone which is separated from said subterranean space by an impermeable seam. The water is transferred to the refill zone using at least one refill boring, and is achieved by flooding said refill zone directly from said subterranean space with said water without first raising the water to the surface.
Another characteristic feature of the present invention involves flooding said refill zone by a pressure less than both the hydraulic resistance between said water storage zone and said refill zone and the lowest rock strength of said impermeable layer between said subterranean space and said refill zone.
The present method further comprises test blasting said impermeable layer and measuring the blasting strength of the impermeable layer and transferring the water to said refill zone at a pressure of at least 5%, preferably 10 to 15% less than said blasting pressure (strength).
In a given case the method may include reducing the refill pressure by methods, such as e.g., acid treatment and/or blasting in said refill borings.
In a further aspect of the present method a packing material or cement material may be placed in at least one bore hole between said water storage zone and said refill zone so as to increase the impermeability of the impermeable layer.
In another feature, the present method may include clarifying said water of imbibition before flooding said refill zone.
In performing the present method at least part of the water of imbibition or the water tapped from the water storage zone by filter wells, in a manner that is already known in itself for protecting the mining operation, is collected preferably in a closed or at least contamination protected gravity collector duct and conducted to a reinjecting pumping station set up in the mine.
Through the reinjecting pumping station, after a treatment, if necessary, the water is refilled through reinjecting borings to the sector of the water storage zone that is separated from the tapped zone of the subterranean water storage system by a zone of high hydraulic resistance and separated from the mine-fields by an impermeable seam with a strength against hydraulic cracking greater than the injection pressure.
In the case of tectonically or by impermeable interlayers cut in subterranean water storage zones, the sufficient hydraulic resistance between the tapped and injected zones can be achieved also by the proper selection of the water storage zone. In other cases the hydraulic resistance between the tapping and refilling zones should be increased by filling the routes of water at least partly as additional measurements.
The safety against hydraulic cracking of the impermeable seam between the mine or other subterranean space and the refill zone of the water storage should be checked by two gauges, namely the pressure gauge of the experimental blasting injection in the impermeable seam and the pressure gauge of the experimental reinjection in the water storage zone.
Depending on measured values, the most unfavorable smallest strength against cracking should be increased suitably by determining the distance between the refill zone and the mine, at most up to the limit according to the original stress condition and/or by reducing the necessary refill pressure either by increasing the number of refill borings or by known methods, e.g. acid treatment or blastings, applied in the reinjection wells.
There may be an extremely favorable water storage to be refilled, the water pressure of which permits refilling by absorption, without pumping.